Frequency-doubling microwave cavity



.United .l States Patent O FREQUENCY-DOUBLING MICROWAVE CAVITY Nick Karayianis, Washington, D.C., and Clyde A. Morrison, Hyattsville, Md., assignors to the United States of America as represented by the Secretary of the Army Filed Feb. 13, 1958, Ser. No. 715,158 3 Claims. (Cl. 321-69) (Granted under Title 35, U.S. Code (1952), scc. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates to microwave devices in general, land more particularly to a microwave cavity device for frequency doubling.

Itis the object of this invention to provide a frequencydoubling microwave cavity device.

`In a typical embodiment, the frequency-doubling cavity device comprises a ferrite loaded ycavity excited in the resonant TMm mode by an input signal of frequency j, with -a D.C. magnetiz-ing lield Iapplied to the ferrite within the cavity. The position of the ferrite, the strength anddirection of the D.C. magnetizing eld, and the cavity dimensions are chosen so that a double frequency component is produced which excites the cavity in the resonant TEM mode, thereby permitting a double frequency signal to be obtained at a properly located output.

The specific nature of the invention, as well as other objects, uses, and advantages thereof, will clearly appear from the fol-lowing description and from the accompanying drawing, in which:

FIGURE 1 is a longitudinal pictorial view of a microwave frequency-doubling cavity device in -accordance with the invention. A portion of the cavity wall is cnt-out to reveal the disposition of the ferrite within the cavity.

FIGURE 2 is a sectional view -along 2-2 in FIG- URE 1.

It has been theoretically derived and experimentally verified that, when a microwave magnetic lield is incident on a ferrite or other insulating ferromagnetic material in a manner such -that the R.F. lield has a component perpendicular to the direction of D.C. magnetization of the ferrite, second order components of magnetic linx at double the R.F. frequency will be produced in the ferrite in the direction of the D.C. magnetization. (See (1) Journal of Applied Physics, vol. 27, February 1956, pp. 18S-189, and (2) Proc. IRE, vol. 44, August 1956, p. 1054.) This second order frequency-doubling effect has heretofore merely been a laboratory curiosity and it was not believed that the effect could practically be employed for frequency doubling. In the present invention, this effect is employed in a novel and practical frequencydoubling cavity device as illustrated in the drawing.

In FIGURE 1, a cylindrical cavity 10 is excited in the TMm niode by a microwave inp'ut at frequency ;f coupled to the cavity 10 by means of a waveguide arm 14 and an iris 17 in a conventional manner. As is well known in the art, the resonant frequency of a cavity excited in the TMm mode is dependent only upon its diameter D and is independent of its length L. This feature is very advantageously used in the present invention as will be brought out later. The field pattern 25 of a cross section in the TMm mode is shown by the dashed lines in FIG- URE 2.

A ferrite rod element 19, preferably having a diameter which is considerably smaller than the diameter of the cavity 10, is disposed along the longitudinal `axis of the cavity 10. Supporting members 22 and 24 of insulating material such as Teflon may be used to support the ferrite element 19. Means are provided, which are also well known in the art, for applying a D.C. magnetizing lield 3,041,524 Patented June 26, 1962 ICC 2 Hd to the ferrite 19 in a direction parallel to the longitudinal axis of the cylindrical cavity 10. The means for applying, yand the magnetizing lield are rrepesented by the symbol Hd c and the arrow 21 in FIGURE 1. The ferrite element rod 19 may alternatively be a sphere, -an ellipsoid, or any other suitable shape.

As con be seen from FIGURES 1 and 2, the R.F. magnetic field at the input frequency f is perpendicular to the `direction of D.C. magnetization Hd c of the ferrite element 19. A-s described previously, this will produce second order components of magnetic liux in the vicinity of the ferrite element 19 at double the R.-F. input frequency, and in a direction Ialong the longitudinal axis of the cavity 10. These second order double-frequency components of magnetic linx are advantageously used in the present invention by choosing the dimensions of the cavity 10, the position of the `ferrite element 19 along the axis, and the strength of the D.C. magnetizing field Hd c, so that the double-frequency magnetic linx excites -a resonant mode in the cavity 10. A series of such possible modes in the cavity 10 are the TEUum modes where n and m are any integers other than zero. These TEOnm modes 'are suitable because they have -an R.F. magnetic field which Ihas a component along the longitudinal axis of the cavity 10. Since the TEOm11 modes are tunable (that is, their resonant frequency Imay be varied) by changing the length L of the cavity 10, the cavity length L is `chosen so that the cavity 10 is resonant at 2f in some TEOmn mode. The position of the ferrite element 19 along the axis is then chosen to excite this TEODLm mode. In the device of the drawing, the length 'L 'of the cavity 10 is chosen to be resonant at the TEM mode. The length L of the cavity 10 may beV conventionally varied by a threaded member I40. An illustrative portion of the R.F. magnetic field in the TEM mode is shown by the dashed lines 30 in FIGURE 1.

Double-frequency energy in the cavity 10 may be conventionally extracted by means of an iris 51 and waveguide arm 57 by locating the iris 51 at a point of strong R.F. magnetic iield in the TEM mode at the double frequency 2f. 'Ilo prevent input frequency energy from -appearing in .the output, it is preferable to have the cutolf frequency of the waveguide arm 57 above the input frequency j. Suitable poi-nts of strong R.F. magnetic lield in the TEM mode are well known in the art.

It is important to note that the operation of the novel cavity device of the drawing is based on the fact that the resonant frequency of the cavity 10 in the TMm mode is dependent only on the diameter and is totally independent of the length L. It is thus possible to adjust the dimensions of the cavity 10 so that the double frequency magnetic linx will excite a resonant TEOmn mode at the double frequency. The frequency at which a cylindrical cavity resonates in a TEOnm mode is dependent upon both the diameter and the length L. Once the diameter has beenchosen so that the cavity 10 resonates in a TMm mode at the input frequency f, the length L is then chosen so that the cavity 10 also resonates in a TE()um mode at the frequency 2f.

I-t is also important to note that even though a cavity has the proper `dimensions to resonate in a particular mode, it will not resonate in that mode unless the mode is excited. In the present invention, excitation in the TEm,m mode is accomplished because it has an R.F. magnetic field component along the axis of the cylindrical cavity. Thus, the ferrite element 19 can be positioned along the axis so that the double-frequency magnetic flux which is produced excites this TEDnm mode.

It has been found that the double-frequency magnetic linx obtained is maximum when the magnitude of the D.C. magnetizing field Hd c is at either of the half-power Y 3 Y Y points'of the' ferromagnetic resonance curve. However, fields slightly further from ferromagnetic resonance should be chosen because as is well known,the Vferrite element 19 absorbs a considerable amount of the input energy the vicinity of'resonancesthereby lowering the elective Q'ofitheiactivity V10.1 The choice ofthe D.'C. niag- Vti'zing eld Hdcg therefore, will be; some Vcompromise value.deper'nient'uponV the magnitude of inputrpower, the Q of the cavity 10, andV Ymatching considerations. The important requirement is that the percentage of the input energyconvertedgto .double-frequency energy be g Those skilled in`the artrwill readily` be able to provide theaproper adjustments for satisfactory opera- -tions p Y Y Y.

A specilic example of a device constructed in accordance withthe; invention for frequency-doubling from about 9000 to 183000'megacy'clesl employsV a cylindrical cavity V1t) having a' diameter of about 1.6 inches and a Ilength L of about :758 inch: Y, The ferrite element 19 comprises a" ferrite-rod abouti 0.2 inch longI with aj diameter of about Olinch. The rod is disposed along the axis of the' cylindrical-cavity about 0.19 inch from oneend. The

' of the'invention asdeined in the appended claims, Y

We ola'imiasour invention: l g Y I Y Y l'. A'requencydoubling microwavey cavity device comfprising incombination: a microwave cavity haw'ng'di-A rme'nsionssuch that the cavity is resonant in the TMm mode atvrthe frequency f andr inV afIEnm mode' at the frequency'Zf, rn'eansffor exciting said cavity in said TMm,

-mode at the frequency f,a ferrite element disposed within saidcavity, a D.C;;magnetizingiield applied trof` said ferrite element, the location'of Vsaid'rferrite element and the Vstrength and-direction ofi saidDfC. field being such Vthat TMzimf modeV energy 'produces double-frequency magnetic 'ilux vhic/:liY excites said' TEdm' mode; and`me`ans for ex-` 4. tracting Ithe double-frequency TEOnm energy from said cantv.. M. ,Y Y f 2. A frequency-doubling microwave cavity device comprising in combination: a microwave cavity having a longitudinal axis, said cavity having dimensions such that the cavity is resonant in the TMmjmode at the frequency f and ina 'IEmmA mode at theA frequency 2f, means'for exciting said' cavity in' `said TMl'm rnode at frequency f, va D.C. magnetizing field applied to said cavity parallel to` said longitudinal axis, a ferrite element disposed within said cavity along said longitudinal axis, the strength of said D.C. eld and' the position' of saidferrite element along said axis being such :thatY TM'm Inode energy produdes double-frequency magnetic linx which excites said TEOIlm mode, and means` foruentracting the double-frequency IEomn mode energy from said cavity.

v V1i.1A.frequency-doubling microwave cavity device comprising a cylindricalV microwave resonan t` .cavity having a longitudinal axis, said cavityhavinga p'iredeterr'ninedV diameter such that the cavity is resonant in the TMm mode at an Vinputfrequency f, means for adjiisting the length of the cavity so that the cavity is simultaneonsly resonant in a TEM@ mode atan output frequency 2f, input waveguide` meansv coupled wto said cavity for exciting said cavity s aid TMm mode, means' for applyingv a D.C. magnetizing field tosaidfA cavity t'alallelv to said longitudinal axis,rferrite means responsive to said D.C. eld

and to TMmym'ode energy for lprodlcing-double-frequeney magnetic lI neansr lfor mounting ferri-tc means lwithin `said cavity along said longitudinal axis so that said `donble-frequency lmagnetic ux excites said ."I'Ef,nm

mode,l andoutput waveguide means coupled to vsaid cavity for extractingthe double-frequency rIYEnmumode energy from said cavity, said outputmeans having a cut-oit frelque'ncy above the input frequency f.

Y ,Referenes cnedra menteur this patent UNITED STATES PATENTS 'zaiskir g f v Mar. 9,' 1954 Y Dobbrtin'.. j;.;.' 1 r- Dec. 24, 1957 Y OTHER' REFERENCES' i Microwave Frequency DonblingfFrom 9 to 18v KMC in Ferrite's ll-Sy I; I3. Melchor, W. PxAyres", and P; H. Va'rtanianrin :Proceedingstof' the I.R.'E. (May 1957) pages 643'f646'relied'on. i Y Y 

